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Old 12-07-2012, 04:52 PM   #1
Cement102
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Default Switching method for 240V element

Hello All,

I am currently constructing an electric HLT using 240 and a heating elelment. I have a programed microcontroller that will be switching the heating element. However, I keep going back on forth on which method I like the best; SSRs or DPST relay/contactor.

SSR: The microcontroller would directly trigger 2 40A SSR (one for each of the 240 loads). I like the simplicity and responce time of doing this. I dont, however like the bleed current. Im not thrilled about the amount of heat Ill have to dissapate too. Has anyone wired their element using solely SSRs? Any thought on these concerns? Has anyone experienced an issue with the bleed current burning the element when the liquid has been transfered out?

DPST Relay or Contactor: I like that this is a mechanical swith that will cut all power when off. However, I will still have to have a secondary relay or SSR to power the coil since my microcontroller is only 5V. It seem this method has more elements, but is a bit safer. Has anyone had issues with the coil burning our durring long duration activations?

So I guess the qustion is: Solely SSRs, or Relay/Contactor?

Thank you in advance!!

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Old 12-07-2012, 05:07 PM   #2
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I run an SSR from a PID, and then I have a (dual pole) Contactor that sits between the SSR and the element. My panel has a toggle switch that I use to control the Contactor.

Code:
                     Contactor              
Line 1 ---------------++++----------------=
                      |  |                Element
Line 2 -----+++++-----++++----------------=
            |SSR|
PID --------+++++
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Old 12-07-2012, 06:02 PM   #3
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Quote:
Originally Posted by abarnett View Post
I run an SSR from a PID, and then I have a (dual pole) Contactor that sits between the SSR and the element. My panel has a toggle switch that I use to control the Contactor.

Code:
                     Contactor              
Line 1 ---------------++++----------------=
                      |  |                Element
Line 2 -----+++++-----++++----------------=
            |SSR|
PID --------+++++
Thanks for reply! This is a similar setup method I have see a lot here. YOu sortof hit the root of my question through. If you are controlling the element using the SSR (i am assuming using temp feedback), why do you need the contator, especially if it is manually switched. I can see it if you are selecting between elements. But for one element, why not just switch the element using the SSR?
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Old 12-07-2012, 06:19 PM   #4
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The SSR is built for high frequency switching, but it can leak current, and can fail closed (on). The normally open contactor by itself is not meant to switch as fequently as the PID would signal it to, however when you switch it off, you know that all power to the element is off. You are using each device to its strengths.

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Old 12-07-2012, 06:26 PM   #5
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Originally Posted by jeffmeh View Post
The SSR is built for high frequency switching, but it can leak current, and can fail closed (on). The normally open contactor by itself is not meant to switch as fequently as the PID would signal it to, however when you switch it off, you know that all power to the element is off. You are using each device to its strengths.
Perfect, that is exactly what I was looking for. So you are using the contactor to "enable" the element and the SSR to actually control it.

So, given that, I would have a contactor that would controll both loads of the 240. The coil would be run off of a 12/24V SSR. This would allow the 5V of the microcontroller to activate the element. Then a second 240V 40A SSR would controll the rapid switching of the element to maintain the control limits of the temp. This allows the Microcontroller to both enable/power the contactor coil and controll the element. Does this sound feasable?

Thanks
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Old 12-07-2012, 07:29 PM   #6
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Another method would be to control the contactor enable/disable via a switch or emergency stop to isolate 220v power from your system. With the contactor, you can use a single SSR for element ON/OFF temp control on one phase of the 220v.

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Old 12-07-2012, 07:32 PM   #7
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Originally Posted by jCOSbrew View Post
Another method would be to control the contactor enable/disable via a switch or emergency stop to isolate 220v power from your system. With the contactor, you can use a single SSR for element ON/OFF temp control on one phase of the 220v.
This would be the safest method. Normally, one leg of 220 is "hot" via contactor leg, and other leg is controlled by SSR - meaning shock hazard between hot leg and neutral/ground.
Plus, if you had SSR controlling contactor, it couldn't keep up with PWM pulses when regulating the element.

edit: this info is in many places. this says it better.
Quote:
Originally Posted by jeffmeh View Post
The PID doesn't generate enough current to run the element, so it controls an SSR that does. The contactor between the SSR and the element allows you to be sure that the element is truly off when you want it to be, given that SSRs can leak and can fail closed.

SSRs and contactors are both relays. The SSR is capable of frequent switching, but has the leakage and fail closed issues. The contactor is not made to switch as fequently as the PID would have it do, but a normally open contactor will be open when you expect it to be. Together they give you what you want, PID control over the element, and a manual switch so you can make sure the element is off when you want it to be.
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Old 12-07-2012, 07:52 PM   #8
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This would be the safest method. Normally, one leg of 220 is "hot" via contactor leg, and other leg is controlled by SSR - meaning shock hazard between hot leg and neutral/ground.
Plus, if you had SSR controlling contactor, it couldn't keep up with PWM pulses when regulating the element.
I was actually planning on using the SSR controlling the contactor as an "enable". This is because there will be a lot of automated operations before and after. So essentially, things happen -> SSR 1 closes the contactor, SSR 2 regulates via PWM, when water heating is done, SSR 1 opens the contactor -> other things happen with no risk of power in the element. So it is kind of like a microcontroller controlled e-stop. (there will be a manual on too )
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Old 12-07-2012, 08:08 PM   #9
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Quote:
Originally Posted by Cement102 View Post
I was actually planning on using the SSR controlling the contactor as an "enable". This is because there will be a lot of automated operations before and after. So essentially, things happen -> SSR 1 closes the contactor, SSR 2 regulates via PWM, when water heating is done, SSR 1 opens the contactor -> other things happen with no risk of power in the element. So it is kind of like a microcontroller controlled e-stop. (there will be a manual on too )
I'll defer to the many good folks here that share their experiences, I'm at the design/acquisition stage.

That said, if I was planning on going wild with automated sequences, I might want to save the limited outputs for something more dynamic than bringing liquid up to heat.
Plus, I'm cheap
Good luck on your build, post some updates with what you end up doing.
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Old 12-07-2012, 08:50 PM   #10
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Quote:
I was actually planning on using the SSR controlling the contactor as an "enable". This is because there will be a lot of automated operations before and after. So essentially, things happen -> SSR 1 closes the contactor, SSR 2 regulates via PWM, when water heating is done, SSR 1 opens the contactor -> other things happen with no risk of power in the element. So it is kind of like a microcontroller controlled e-stop. (there will be a manual on too )
You can do it that way, I have a Manual 3 way switch that controls which contactor is getting power and restricts my system to only having one element on at a time. I like this because I have to be physically present to power up an element. I have a BCS-460, and have turned it on in the past and have had it signal every thing to be on at the same time. I have float switches in line to the SSR so the elements did not fire. For you If you had a similar thing happen the SSR you have to control the contactors would both be on and so would both elements, if I am reading your post right.
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